The present invention relates to a piston assembly for an internal combustion engine, and, more particularly, relates to a piston assembly for an internal combustion engine which incorporates an oil cooling system for cooling the piston during operation of the engine.
A piston assembly for an internal combustion engine generally becomes very hot during use, and is subjected to relatively severe thermal stresses as compared to other engine parts, especially on its top wall or crown portion which is directly exposed to the heat of the gases in the combustion chamber partly defined by the piston. This problem of heating of the crown of the piston assembly has become more and more severe with modern internal combustion engines, due to increases in thermal load on the parts thereof arising from increases in engine power output. Various schemes have been developed in the past for aiding with the cooling of such a piston assembly; and nowadays some form of active cooling for the piston assembly is coming to be quite necessary.
In particular, the concept of cooling the piston crown from below by injecting a flow of engine lubricant from the crank chamber side up into the space defined by the cup shaped piston structure including the piston crown and the piston skirt, so as to impinge against the lower side of the piston crown and to cool it, has been put forward in the past in various forms: for example, such piston cooling constructions have been proposed in Japanese Utility Model Application No. 38-11185 (Publication No. 40-19201), Japanese Utility Model Application No. 42-101852 (Publication No. 45-32981), Japanese Utility Model Application No. 49-96796 (Publication No. 54-26424), Japanese Utility Model Application No. 55-42967 (Laying Open Publication No. 57-156052), Japanese Patent Application No. 58-138183, Japanese Utility Model Application No. 58-164040, and Japanese Utility Model Application No. 58-188456. And, in particular, it has been recognized that it is helpful for such lubricant cooling of the piston crown to provide a member near the lower surface of said piston crown which defines a reservoir for temporarily and intermittently accumulating a pool of lubricant therein, so that lubricant from this pool can be splashed against the piston crown as the piston reciprocates in the cylinder bore.
Such a lubricant reservoir may be defined by a part of the piston assembly which is integrally formed or cast with the piston main body itself, or is welded thereto; but this presents difficulties such as increasing difficulty and cost of manufacture and introducing quality problems during manufacture. Because of this, in the above identified applications, there has been proposed the concept of providing this lubricant reservoir as defined by a shelf plate member fixed in the space within the piston main body near the piston crown. However, the prior art methods which have been proposed for securing such a shelf plate member within the piston main body, such as for example securing it with screws, or by fixing it by the spring action of a retainer spring, have been unsatisfactory. In the case of fixing by screws or other fastener members, the vibration, thermal stresses and changes, and accelerative forces to which a piston assembly is subject during operation of the engine make it almost impossible to guarantee that such fasteners should not become unfastened; and in the case of securing with a retainer spring the reliability of fastening is not good, and risks occur of loss of spring action of the spring being caused by high operating temperatures, and of vibration again causing detachment of the shelf plate member.